Low alkali, non-crystalline, vitreous silica fillers

ABSTRACT

A substantially white powder for use as a filler and/or extender derived from by-products of manufacturing vitreous low alkali, low iron glass fibers, and a method for producing the powder. The filler has very low alkalinity and by virtue of its being essentially free of crystalline silica is non-hazardous to health and therefore safe for consumer-based and industrial-based uses.

RELATED APPLICATIONS

This application is a continuation-in-part of our application Ser. No.10/873,470 filed Jun. 21, 2004, now U.S. Pat. No. 7,070,131, which is adivisional of Ser. No. 10/087,064 filed Mar. 1, 2002, (now U.S. Pat. No.6,776,838) which claimed priority from Provisional application No.60/273,176 filed Mar. 2, 2001.

FIELD OF THE INVENTION

The present invention relates generally to fillers and extenders, andmore specifically relates to a white filler and extender derived fromglass manufacturing by-products, and to the method for producing thesaid product. The product is a non-crystalline, vitreous aluminosilicateof low alkali content and high brightness and finds application as afiller and extender in plastics, paints, coatings, and in other commonuses for fillers and extenders.

BACKGROUND OF THE INVENTION

In a representative glass fiber manufacturing facility, typically 10-20wt % of the processed glass material is not converted to final productand is rejected as industrial by-product or waste and sent for disposalto a landfill. This rejected material represents a substantial cost tothe industry and also generates a consequent detrimental impact on theenvironment. While the rejected by-product referred to may have widelyvarying physical form, ranging from thick fiber bundles to partiallyfused fiber agglomerates and shot, it is evident from chemical analysesof various samples recovered at different times, that the material stillhas a substantially constant chemical and mineralogical make-up. Thus,unlike wastes from many other industrial processes which typically havewidely varying chemical and mineralogical properties, the waste from theglass fiber manufacturing process is very consistent in composition andstill benefits from the stringent engineering quality control applied tothe glass-making process itself. This consistency is a major advantageto any potential utilization of the glass fiber manufacturing waste.

More specifically, the glass formulations of great relevance to thisinvention are those of low alkali calcia-alumina-silica compositions(CaO—Al₂O₃—SiO₂ or “CAS”) typically used for commercial glass fibermanufactured to comply with ASTM D-578. These formulations are given inTable 1. The compositions are vitreous and by virtue of their componentshave very low levels of discolorants. These compositions are expressedconventionally in terms of the element oxide and are not meant to implythat the oxides, crystalline or otherwise, are present as distinctcompounds in the amorphous glasses.

TABLE 1 Composition Range Component (Element Oxide) (% by Weight)Silicon dioxide, SiO₂ 52–62 Aluminum oxide, Al₂O₃ 12–16 Iron oxide,Fe₂O₃ 0.05–0.8  Calcium oxide, CaO 16–25 Magnesium oxide, MgO 0–5 Sodiumoxide + potassium oxide (Na₂O + K₂O) 0–2 Boron oxide, B₂O₃  0–10Titanium dioxide, TiO₂   0–1.5 Fluorine, F₂ 0–1 MineralogicalComposition (XRD) Amorphous (glassy)

Several features are immediately evident from inspection of the data inTable 1. First, the general chemical and mineralogical composition ofthe glass fiber material is very similar to amorphous (glassy) calciumalumino-silicate materials, such as blast-furnace slag and Class C flyash, that are commonly used as cementitious or pozzolanic admixtures inportland cement concrete; second, the alkali (Na₂O+K₂O) content of theglass is very low (0 to 2%); and third, with their inherently low ironcontents (0.05 to 0.8%), the glasses have little or no color. Low alkalicontent and chemical consistency differentiates the glass fibermanufacturing waste from post consumer waste glass, for examplecontainer bottles and flat glass, that have widely varying chemicalcomposition, generally high alkali content, and in the case ofcontainer/bottle glass are highly colored.

SUMMARY OF INVENTION

In the aforementioned parent patent applications the present inventorshave found that once it is ground to a powder of suitable fineness, theglass fiber waste discussed above can effectively function as a reactivepozzolanic admixture for use in portland cement-based building materialsand products, such as concrete, mortars and grouts. In another distinctaspect of the present invention, however, it has been found that thesepowder products can also serve as outstanding fillers and extenders inthe manufacture of plastics, paints, coatings and in other conventionaluses of fillers and extenders.

The finely ground glass powder of the invention (which retains thevitreous nature and chemical composition of the fiber feed) is white incolor having a brightness as high as 90 or more (were measured asdiscussed below). The product is entirely vitreous, and thus containsessentially no crystalline silica. This is an exceedingly significantproperty, since it renders the silica based material safe for use inconsumer and industrial applications in contrast with the healthhazardous crystalline silica fillers of the prior art. These safeproperties also assure that recycling of materials containing suchfiller/extenders will not be hampered by the presence of a hazardousfiller. Furthermore the very low alkali content minimizes theaccumulation of the alkali bloom phenomenon which is common with manyprior art fillers. The products of the invention when used e.g. asfillers in polymers can be loaded in the polymer to typical levels of 20to over 60% by weight. Depending upon loading and the specific polymerinvolved, desired mechanical, thermal and/or electrical properties ofthe filled polymer can be achieved. Because the fillers have low oilabsorption, lower viscosities are present during manufacture,facilitating, processing of the filled polymer. Furthermore the lowalkalinity of the fillers leads to greater stability in the filledmaterials.

According to a process aspect of this invention, glass fiber wastes areconverted into high quality filler and extender products, by shreddinglong entangled strands of glass into short fibers, adjusting themoisture content of the short fibers, grinding the short fiber, andclassifying the ground material to produce a uniform high qualityproduct with precise control over the maximum particle size and particlesize distribution. Because of its physical characteristics, this productwill at times herein be referred to as “white VCAS filler/extenders”,the “VCAS” being a reference to its production from fibers of “vitreouscalcium-alumino-silicate” glass. The white VCAS filler/extender has areflectance value of at least 80 as measured by a Technibrite TB-1Ccolorimeter according to the ISO 2467, 2471 method, and as alreadymentioned can have brightnesses of 90 or even higher.

BRIEF DESCRIPTION OF DRAWINGS

In the drawing appended hereto:

FIG. 1 is a schematic block diagram illustrating a process which may beused to prepare the fillers and extenders of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

According to the process aspect of this invention, glass fiber wastesare converted into high quality filler and extender products, by ashredding long entangled strands of glass into short fibers, adjustingthe moisture content of the short fibers, grinding the short fiber, andclassifying the ground material to produce a uniform high qualityproduct with precise control over the maximum particle size and particlesize distribution.

The process of glass manufacturing entails melting a mixture ofcarefully selected oxides, then cooling the molten material to producethe desired size, shape, and characteristics (e.g., container glass,flat glass, optical glass, fiber glass, etc.). The carefully selectedingredients for glass manufacturing are typically based on specificformulations of three material types: i.e., glass formers, glassmodifiers or fluxes, and stabilizers. Glass formers comprise the majorcomponents of glass and most commonly consist of silicon dioxide in theform of sand and aluminum oxide in the form of alumina. Boron oxide isanother common glass former component found in some formulations. Glassmodifiers or fluxes lower the melting temperature and alter theviscosity of the glass melt and consist primarily of alkaline earthmetal and alkali metal oxides, typically derived from the raw materialscalcium carbonate, sodium carbonate and potassium carbonate. Stabilizersare added to make the glass strong and resistant to water and chemicalattack. Low alkali glass, such as many of the formulations typicallyused for the manufacture of high performance glass fiber, is speciallyformulated for resistance to high temperatures and corrosive substances,in addition to having high physical strength and flexibility.

The process of glass fiber forming involves feeding molten glass from ahigh temperature furnace through a series of bushings containingthousands of accurately dimensioned holes or tips. Fine individualfilaments of glass with diameters typically in the range 20-60 micronsare drawn mechanically downward from the bushing tips, cooled andbrought together to form bundles or strands of glass fibers. In theprocess of forming glass fibers, a significant amount of wastage isgenerated, mostly in the form of irregular, entangled long strands andbundles, often with nodules from partial fusion. The waste strands andbundles can be many tens of feet in length and are in a form that is notconducive to easy handling and processing by conventional means. Thiswaste material is typically cooled by water and air quenching andshipped to a landfill for disposal. According to this invention a largeamount of this waste glass fiber material can be processed and convertedinto high performance industrial products.

A typical process useful in the invention is shown schematically inFIG. 1. In the first step of the present process, the glass fiber waste(feed stock) is collected and placed in a containment area forde-watering and trash removal. Water used to cool the waste fiber streamis allowed to drain off the fibers and is collected and transferred tothe wastewater treatment system. Incidental trash objects are manuallyremoved from the bulk waste materials to allow for further processing.

In the second step of the process, the moist waste glass fiber bundlesare processed by a shredder at fiber shredding through a shredder toreduce the fiber length from infinitely long entangled strands to shortfibers (typically less than 10 mm) for subsequent processing. Theshredding stage consists of processing the entangled strands through arapid rotating mandrel with protruding cutting knives. Stationarycutting edges are also located opposite the rotating mandrel. The fastcutting action of the knives snaps the entangled glass bundles andstrands into the desired short individual fibers. A screen enclosurearound the rotating mandrel is used to retain the large entangledstrands and ensure shredding into short fibers.

In the third step of the process (fiber drying), the moisture content ofthe shredded short fibers is adjusted prior to further processing usingdry and heated air. The moisture content is controlled to apredetermined specific range to optimize the subsequent grindingprocess. Generally the moisture content should be less than 10% byweight, and is preferably less than 2% by weight. In a very typicalinstance the moisture content is from 0.5 to 1.0% by weight.

In the fourth step of the process, the shredded short fibers aresubjected to fine grinding by being processed through an attrition mill,preferably in a vertical attrition mill such as a stirred or agitatedball mill. The short fibers and the ground glass are very abrasivematerials. Abrasion of commonly used stirred mill components not onlycontaminates the product, it also reduces the grinding efficiency. Inthe present process the mill uses a rotation shaft and arms that agitatethe grinding media and create both impact and shearing action, resultingin efficient product size reduction. The rotating arms are covered withreplaceable leading-edge ceramic protectors composed of die cast andheat-fused alumina. The wall of the attrition mill is also lined withabrasion-resistant alumina to further minimize product contaminationfrom the metal components in the mill. The mill uses the highest qualityhigh alumina grinding media consisting of ⅛″ to ⅜″ diameter balls. Theeffectiveness and efficiency of the attrition mill are greatly enhancedby the die-cast, heat-fused leading edge protector attachments of theagitator arms. Energy inputs used in this grinding process are at least100 kW-hrs/ton of feed fibers and typically are in the range of 100 to200 kW-hrs/ton of the feed fibers.

The attrition mill is typically operated with continuous feed anddischarge, although if desired it can alternatively be operated in abatch mode. The discharged grinding media and product are separated instage five of this process using a vibratory screen with 80 to 100 meshopenings. The grinding media and oversize glass comminution products arereturned to the attrition mill for continuous processing. The groundglass product passing the screen is conveyed to an air classificationsystem for product refinement.

In step six of the invention (fine powder classification), the groundglass product is processed through a high-performance, dual-cyclone, dryair classification system. This stage is used to control the finenessand particle size distribution of the product from fine grind tolow-micron range depending on the required specification. Particleslarger than the maximum allowable are returned to the attrition mill forfurther grinding. The use of an air classification system in this stageallows for precise control over the maximum particle size and ensuresthe production of a uniform product. The air used in classification isvented through a filter fabric dust collector (Air emission controlsystem). Ultra fine particles collected in the filter fabric can beblended with the final product (Blending Packaging).

The final classified white filler or extender product will generallyhave a particle size distribution such that at least 95% of theparticles by weight have an equivalent spherical diameter (E.S.D.) ofless than 45 μm (microns). Typically 95% by weight may be less than 25μm; (typical median size around 9 μm); and for many applications themilling and classification will provide an end product where 95% byweight of the particles are of less than 10 μm E.S.D. (a typical mediansize here is around 3 μm); and in other instances the said end productcan have P.S.D.'s where 95% of the particles by weight are less than 5μm, or even less than 3 μm.

The finely ground white VCAS filler/extenders product as produced bythis process is characteristically of a blocky, almost equi-dimensionalparticle shape, with no evidence of residual high aspect ratio fibers.The aspect ratio of the particles will typically average less than 2:1,with the aspect ratio becoming smaller as the average particle sizebecomes smaller as a result of the milling and classification asdiscussed above. The finely ground powder product yielded by theinvention can be packaged in bags or sold in bulk for industrial fillerand applications. This product can serve as a replacement to high pricedwhite fillers and extenders. The final product from the process containssubstantially no particles which NIOSH defines as “respirable fibers,”i.e., particles which are greater than 5 μm in length and less than 3 μmin diameter with an aspect ratio of greater than or equal to 5:1.

The invention is further illustrated by the following Example, which isindeed to be considered exemplary of the invention, and not definitivethereof.

EXAMPLE 1 Preparation of VCAS Filler/Extenders

To facilitate an evaluation of their properties, by-product glass fiberwaste materials having compositions as shown in Table 1 were ground tofine powders with a variety of different particle size distributions orfinenesses. This was carried out using both laboratory and pilot-scaleequipment in a multi-stage process involving drying, comminution,screening, and high efficiency air classification, the object being tohave no residual high aspect ratio particles (shards) in the powderproducts. Representative sub-samples of the ground product materialsfrom this process were characterized for their granulometry properties,some illustrative examples of which are shown in Table 2.

TABLE 2 SSA Median D95 Pozzolan ID (m²/kg) (μm) (μm) GP1 269 nd 50 GP2560 12 30 GP3 580 10 30 GP4 686 9 25 GP5 788 6 20 GP6 956 3 10 GP7 >12001 3

The specific surface area (SSA) of the powders was determined by theBlaine air permeability method according to ASTM C-204. The results inTable 3 show that the range of specific surface areas for the preparedVCAS pozzolan powders was 250 to greater than 1200 m²/kg. Thecorresponding particle size distribution, median particle size, and D95(particle size with 95% of the particles finer) of the products, weredetermined by the laser interferometer technique in aqueous dispersionusing Microtrac® X100 or Coulter LS® particle size analyzers. The medianparticle sizes of the VCAS filler/extender products ranged from 1 μm(microns) to 12 μm, with corresponding D95 values ranging from 3 μm(microns) to 50 μm. The specific gravity of the VCAS filler/extenderpowders, as determined by the Le Chatelier method (ASTM C-188), was 2.57cm²/g.

Examination of the VCAS filler/extender powders at high magnification byscanning electron microscopy (SEM) confirmed that, as is typical of suchground materials, all the VCAS pozzolan samples were substantiallyblocky in particle shape. There was no sign of residual high aspectratio particles. X-ray powder diffraction (XRD) analysis of the powdersconfirmed that that they were all essentially amorphous in structure.

While the present invention has been described in terms of specificembodiments thereof, it will be understood in view of the presentdisclosure, that numerous variations upon the invention are now enabledto those skilled in the art, which variations yet reside within thescope of the present teaching. Accordingly, the invention is to bebroadly construed, and limited only by the scope and spirit of theclaims now appended hereto.

1. A white brightness powder for use in filler and extender applicationscomprising a finely ground vitreous calcium aumino-silicate(CaO—Al₂O₃—SiO₂) powder derived from previously discarded glass fibershaving low alkali and low iron content and low levels of discolorants;said powder having an alkali content by weight of not more than 2% as(Na₂O+K₂O), an iron content by weight of not more than 0.8% as Fe₂O₃ andbeing white in color with a reflectance value of at least 80 as measuredby a Technibrite TB-1C colorimeter according to the ISO 2467, 2471method; at least 95% by weight of the powder comprising particles ofless than 45 microns E.S.D. and said particles having a blocky,equi-dimensional particle shape wherein the average aspect ratio of theparticles is less than 2:1, and with substantially no residual highaspect ratio fibers.
 2. A composition in accordance with claim 1,wherein the said powder has a P.S.D. such that at least 95% by weight ofthe particles are of less than 25 microns E.S.D.
 3. A composition inaccordance with claim 1, wherein the said powder has a P.S.D. such thatat least 95% by weight of the particles are of less than 10 micronsE.S.D.
 4. A composition in accordance with claim 1, wherein the saidpowder has a P.S.D. such that at least 95% by weight of the particlesare of less than 5 microns E.S.D.
 5. A composition in accordance withclaim 1, wherein the said powder has a P.S.D. such that at least 95% byweight of the particles are of less than 3 microns E.S.D.